Many methods exist for objectively measuring the perceived loudness of audio signals. Examples of methods include A-, B- and C-weighted power measures as well as psychoacoustic models of loudness such as described in “Acoustics—Method for calculating loudness level,” ISO 532 (1975) and said WO 2004/111994 A2 and US 2007/0092089 applications. Weighted power measures operate by taking an input audio signal, applying a known filter that emphasizes more perceptibly sensitive frequencies while deemphasizing less perceptibly sensitive frequencies, and then averaging the power of the filtered signal over a predetermined length of time. Psychoacoustic methods are typically more complex and aim to model better the workings of the human ear. Such psychoacoustic methods divide the signal into frequency bands that mimic the frequency response and sensitivity of the ear, and then manipulate and integrate such bands while taking into account psychoacoustic phenomenon, such as frequency and temporal masking, as well as the non-linear perception of loudness with varying signal intensity. The aim of all such methods is to derive a numerical measurement that closely matches the subjective impression of the audio signal.
The inventor has found that the described objective loudness measurements fail to match subjective impressions accurately for certain types of audio signals. In said WO 2004/111994 A2 and US 2007/0092089 applications such problem signals were described as “narrowband”, meaning that the majority of the signal energy is concentrated in one or several small portions of the audible spectrum. In said applications, a method to deal with such signals was disclosed involving the modification of a traditional psychoacoustic model of loudness perception to incorporate two growth of loudness functions: one for “wideband” signals and a second for “narrowband” signals. The WO 2004/111994 A2 and US 2007/0092089 applications describe an interpolation between the two functions based on a measure of the signal's “narrowbandedness”.
While such an interpolation method does improve the performance of the objective loudness measurement with respect to subjective impressions, the inventor has since developed an alternate psychoacoustic model of loudness perception that he believes explains and resolves the differences between objective and subjective loudness measurements for “narrowband” problem signals in a better manner. The application of such an alternative model to the objective measurement of loudness constitutes an aspect of the present invention.